Wellbores serve, in particular, for discovering crude oil or natural gas and extracting it from subterranean deposits. The latter extend in horizontal and vertical direction, often over an extensive area, so that a plurality of wellbores are used for each individual deposit in order to recover the crude oil or the natural gas from the deposits.
In this process, each wellbore has tubing made up of a plurality of individual pipes, vertically adjoining one another, which, when the wellbore is introduced, are inserted one after the other and joined to one another. These pipes of the tubing extend down to and into the oil or gas deposit.
This means that the desired medium, that is, in particular, crude oil or natural gas, is located in the deepest region of the wellbore and can be extracted. For this purpose, there are pumps or other extracting devices in this region, which bring about a continuous or discontinuous upward extraction within the wellbore. Further quantities of crude oil or natural gas flow from the sides and from below to replace the withdrawn medium. In addition, water and/or other fluids accumulate in this lowest region of the wellbore. The surface of the fluids in relation to the ground surface is referred to as the fluid depth.
The adjoining pipes of the tubing of the wellbore are joined to one another in such a manner that a relatively tight, mechanically stable joint is formed and, in particular, no sand and no fluids can penetrate through the wellbore tubing into the interior of the wellbore. In this case, the pipes have pipe joints to one another, that is, coupling sites that are formed by the collars and coupling rings. The individual pipes are essentially roughly equal in length.
Often provided for technical reasons are two tubings that are put in place concentrically in each other. The crude oil or natural gas to be extracted flows upward in an inner pipe and, in an annulus concentrically surrounding the inner pipe from the outside, there takes place, among other things, a pressure equalization.
The precise location of the fluid depth in a wellbore has already long been of appreciable interest for improving the extraction operation. For example, it is undesirable for the oil pumping operation to withdraw more crude oil from the wellbore than can flow back into this wellbore from the deposit. In end effect, the wellbore would then run dry. Naturally, an effort is also made to withdraw as much crude oil or natural gas from the wellbore as possible. Because the rate at which the crude oil or natural gas flows back cannot be measured and, over the operating time of a wellbore, varies in an unforeseeable manner, the exact location of the fluid depth is a value of interest. If the fluid depth drops over the course of time during each of a number of observations, the fluid level also drops, so that it appears that more crude oil and natural gas is being withdrawn from the wellbore than can flow into the wellbore from the deposit. This would pose a possible danger as extraction continues. On the other hand, if the fluid depth rises, there exists the possibility of increasing the rate of extraction.
The location of the fluid depth in the wellbore can naturally be several hundred or even greater than 1000 m deep and is accordingly hard to log and determine from the ground surface. Optical measurements are ruled out in this case.
For the measurement of the water level in a wellbore, which serves as a well for extracting water, an acoustic measurement is proposed in U.S. Pat. No. 5,027,655. A signal having a known frequency is transmitted downward into the wellbore and a returning signal, reflected from the surface of the fluid, is utilized as a response signal. Taking into consideration the speed of sound, it is possible to determine the fluid level. In this case, the wellbore may not have any major discontinuities that act as strong reflectors for the emitted signal and therefore lead to interference with the result. U.S. Pat. No. 5,027,655 proposes, as a further modification, a variation of the frequency in order to determine a resonance frequency or an antiresonance frequency and to deduce from it the location of the water level.
Undertaken in practice are acoustic measurements such as described, for instance, in U.S. Pat. No. 4,793,178 and U.S. Pat. No. 4,934,186 as well as, in further developments, also in U.S. Pat. No. 5,117,399 and U.S. Pat. No. 5,200,894. A gas cannon is fired on the ground surface in the region of the wellbore and, in this way, generates at acoustic impulse, which has a very steep and high front flank.
Alternatively known, besides the firing of a gas cannon, is the generation of an impulse by implosion from a device offered by Friedrich Leutert GmbH & Co. KG under the name “Acoustic Well Sounder AWS.” This method is suitable when, in the wellbore, there exists an overpressure with respect to the surrounding air. To this end, the uppermost section of the tubing in the wellbore is closed off by a pipe that can be shut at the upper end and at the lower end. In the first measurement step, the upper valve is then opened and the gas contained in the pipe escapes into the surroundings. Then, the upper shutoff valve is closed again. Afterwards, the lower shutoff valve in the wellbore is opened and the gas present under overpressure in the wellbore enters the pipe abruptly. This abrupt expansion of the gas present in the wellbore into the pipe also generates an impulse having a very steep and high front flank. Then, in the next step, the gas that is now contained in the pipe from the wellbore is expelled into the surroundings and the next measurement can take place.
The pressure wave generated by the firing of the gas cannon or else by the described implosion travels down through the wellbore to the fluid depth and is then reflected there. In this process, it is subject to the physical laws of sound waves.
However, the speed of sound in the wellbore is not known, because a complicated gas mixture, which, moreover, is not constant in time and which consists of, among other things, methane, nitrogen, carbon dioxide, argon, and many other gases, is present in the wellbore between the ground surface and the fluid depth and behaves in no way like an ideal gas and also changes in composition both with time and over the depth of the wellbore in an unforeseeable manner. This means that it is not possible to deduce automatically the distance of travel of a sound wave from its time of travel and it is not yet possible to make a quantitative statement about the distance of the path traveled from the time of travel of the sound wave from the ground surface to the fluid depth and, after reflection, back to the ground surface. Hence, the fluid depth cannot be determined only from the time of travel.
Also, smaller reflections take place at the individual joints of the pipes of the wellbore tubing, so that portions of the sound or pressure wave are reflected back even before the reflection from the fluid surface and can be recorded above at the ground surface by using an appropriate receiving device. These waves thus have a shorter distance of travel and hence also a shorter time of travel.
However, because the individual pipes are, as mentioned, of roughly equal length and the discrete reflections at the edges of the joints can be observed, an entire series of reflection lines of sound waves result, before the impingement of the sound wave reflecting from the fluid level, and these sound waves have reached only the individual pipe joints, that is, the coupling sites between the pipes, and been reflected there. If these reflecting sound waves can be identified and adequately clearly detected in the noise, then, by subtracting the reflection lines, it is possible to determine how many pipe joint lines lie between the ground surface at the wellbore and the fluid depth. With a known average pipe length, it is then possible to deduce the approximate location of the fluid depth and, by comparison of several measurements at intervals in time, it is also possible, in a later analysis, to deduce whether the fluid depth is rising or falling.
A problem is that there are multiple reflections, because each sound wave that reflects at a joint and travels upward is partially reflected anew at all of the above-lying joints and these doubly reflected sound waves travel downward and result in additional triple and multiply reflected sound waves that impinge at the ground surface due to renewed reflection and interfere with and appreciably hamper the identification of the reflection lines that are of actual interest. Hence, when the gas cannon is fired, a great deal of care must be applied to achieve an initial sonic event that is as clean as possible, that is, to obtain a flank that is as steep and as high as possible, which enables a later identification of desired and undesired reflection lines.
Proposed in U.S. Pat. No. 6,085,836, in view of this complicated situation, is to employ three separate signal generators, each of which emits a signal of a certain frequency, with the three frequencies being different from one another. These three signal generators are intended to then emit the sound waves generated by them separately from one another, first of all within a drill string, then outside of a drill string, and, finally, additionally in an annulus. Through reception of the reflected signals in the various wellbore regions and the assignment of the monofrequency sound waves to the initial signals, it is hoped to improve the measurement results and to make the measurements more reliable. The expense for signal generators that operate separately from one another and operate in separate regions of the wellbore cross section is appreciable. In practice, this proposal has not gained acceptance.
Besides the difficulties already discussed in the analysis of reflecting sound waves, it is desirable to prevent all interfering noise due, for instance, to running extraction pumps and the like, which could also interfere with the measurement result or falsify it. Thus, in such measurements using methods of prior art, it is necessary to interrupt the drilling operation for the time of the measurement in many cases.
In practical application, such an arrangement of devices involving a gas cannon and a receiving device as well as an associated electronic analysis and evaluation device is then employed in succession at the various wellbores of an oil field or other deposit. The analysis then takes place largely manually after all measurements have been concluded. From the resulting measured values, experts can then, insofar as required undertake measures at the individual wellbores to adjust the rate of extraction, for example.
In spite of the fundamental functional ability of such methods, there exists the desire in expert circles to be able to undertake this operating procedure at lower cost and, insofar as possible, also faster. To be taken into consideration in doing so is the fact that technical personnel are needed over a prolonged period of time for the operation in order to appropriately carry out and analyze measurements from a large number of wellbores. The arrangements of prior art are exacting and have to be used in places where the wellbores are located, which is often the case in impassable terrain that is not favorably located for vehicle access.
The problem of the invention is therefore to propose a class-specific method and a class-specific device by means of which an improved or simplified determination of the fluid depth in wellbores is possible.